The present invention relates to a method for treating an object manufactured, in particular by additive manufacturing, from a material having a defined melting temperature, wherein the treatment comprises subjecting the object to hot isostatic pressing for reducing the porosity and increasing the density of the object.
Generative or additive manufacturing methods are increasingly being used to produce prototypes or completed components very quickly. In contrast to conventional production processes, which comprise removing material from a block of material by, for example, milling, cutting, drilling or other machining processes, additive manufacturing methods construct a desired three-dimensional object directly layer by layer based on a digital description or representation of the object. They are also known as 3D printing or rapid prototyping.
Specific examples for additive manufacturing methods are the so-called selective laser melting (SLM) and the so-called selective electron beam melting, in which a laser beam and an electron beam, respectively, is used to irradiate and melt selected portions of layers of materials, which are successively applied to a support. Another example is the so-called fused deposition modeling, in which a heated and movable extrusion nozzle dispenses molten material to build the desired object. Generally, many additive manufacturing methods are based on melting or fusing and solidifying a meltable material, in particular a metal or thermoplastic material.
Irrespective of the specific type of additive manufacturing method, as explained above, the object is constructed directly layer by layer in a three-dimensional manner. This makes it possible to produce different highly complex objects efficiently and quickly in the same device from different materials, in particular from metal but also from plastic materials, and in particular thermoplastic materials, and ceramic materials. For example, highly complex grid or honeycomb structures or other highly complex structures which cannot be generated, or can only be generated with difficulty, using other processes can be easily produced. In comparison with traditional production processes, the complexity of the object has only a limited influence on the production costs.
Additive manufacturing, combined with topology optimized design, provides an opportunity to save weight for structures like aircraft. In particular, an optimization as to minimum weight is possible while taking into consideration requirements such as stiffness, strength and stability.
However, objects manufactured by additive manufacturing typically have a relative high porosity and a relative high surface roughness. It is desirable to reduce the porosity and the surface roughness as much as possible in order to further improve the fatigue properties of the object manufactured. The same may also be true for objects manufactured by other methods.
Hot isostatic pressing is a known process for reducing the porosity of objects to thereby improve the mechanical properties and the workability of an object. For this purpose, the object is arranged in a pressure chamber and subjected to both an elevated temperature and a high isostatic pressure, for example by arranging the object in a heated and pressurized liquid, such as oil. Since the high isostatic pressure is applied to the object from all directions, the object is compacted without changing its shape.
While various processes for reducing surface roughness are generally known, they are difficult to apply to objects having a complex shape, in particular if they have interior surfaces which are difficult to access.
Typical known surface smoothing methods involve mechanical grinding or polishing using an abrasive medium, chemical or electrochemical polishing, or the use of abrasive liquids. Hollow structures manufactured using additive manufacturing have interior surfaces which can hardly be reached by these methods, in particular if the hollow structures are divided into interior subspaces by walls or other portions of the structures. Only electrochemical polishing or abrasive liquids have the potential of providing sufficient access to hollow structures. However, electrochemical polishing tends not to produce homogenous results, because the distance between anode and cathode varies, and abrasive liquids cannot reach all small corners evenly or do have the same level of abrasion across the whole surface depending on the structural layout.